This invention relates to an apparatus for supporting and rotating a tubular member mounted on a lathe, and, more particularly, to such an apparatus for use in the MCVD process for producing optical fiber
Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective glass cladding. Presently there are several known processes for fabricating preforms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027, issued in the names of J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Telephone Laboratories, Inc., has been found to be one of the most useful because the process enables large scale production of preforms which yield very low loss optical fiber.
During the fabrication of preforms by the MCVD process, reactant-containing gases, such as SiCI4 and GeCI4 are passed through a rotating substrate or starter tube suspended between the headstock and tailstock of a lathe. A torch assembly, which heats the tube from the outside as the gases are passed therethrough, traverses the length of the tube in a number of passes, and provides the heat for the chemical reactions and deposition upon the inner wall of the tube. The torch assembly also supplies the heat for collapsing the tube to form a rod, and, in subsequent operations, for collapsing an overclad tube onto the rod, as explained in U.S. patent application Ser. No. 09/353,943 of Mueller et al., filed Jul. 15, 1999. In the current manufacture of preforms, the torch is mounted on a carriage which is a solid structure supported and guided on the lathe or machine bed. In such a system, it is desirable that operation of all moving parts be smooth and uniform but there are several areas in which such smooth operation is difficult to achieve. Thus in the systems as currently used, sliding or rolling elements on the carriage are in direct contact with the bed of the lathe or machine or with the ways. In all such systems, the movement of the carriage and the physical contact between it and the bed requires lubrication to eliminate wear and friction. An initial xe2x80x9cstick-skipxe2x80x9d condition must be overcome during the start of carriage motion which is a result of the friction, and the friction can also induce xe2x80x9cjerkxe2x80x9d in the movement of the carriage along the bed. In addition, the friction can cause or induce, over a period of time, freeplay in the system as a result of wear. Thus, where a smooth uniform velocity of the torch down the length of the tube virtually is a necessity for uniformity of heating and deposition and, ultimately, a uniformity of product, the friction effects can, and most often do, cause a non-uniform velocity profile, and, as a consequence, non-uniformity of heating and deposition, which result in non-uniformity of product. In present day practice, friction is overcome, at least in part, through the use of lubricants which, during a production run, become a contaminant to the process and spread throughout the machine. This, in turn, necessitates frequent cleaning of the apparatus which is detrimental to the goal of substantially continuous production. Further, the lubricant does not completely eliminate the stick-slip and jerk problems which, as pointed out in the foregoing, most often lead to a non-uniform velocity profile.
The related U.S. patent application Ser. No. 09/500,154 of Mueller is directed to a carriage guidance system that substantially eliminates physical contact between the carriage and lathe bed and, hence, overcomes most if not all of the aforementioned problems. The arrangement shown in that application is a hydrostatic guidance and support system for the movable carriage upon which the torch for the MCVD process is mounted. The carriage, as used on the MCVD lathe, is equipped with integral air bearing components which, in their geometry, match the lathe bed cross-section. Fluid, such as air, under pressure, is delivered to the bearings which, under pressure of the air or whatever fluid is used, in use, cause the carriage to float in spaced relationship to the lathe, thereby producing a substantially friction free support and guide for the carriage, which results in a smooth velocity profile, which, in turn, produces a drastic improvement in the quality (and quantity) of the MCVD product.
There remains, however, in present systems, another source of non-uniform or jerky movement, and that is in the drive system of the lathe for rotating the tubular member or members. The headstocks of the lathe provide the means for rotating, for example, the preform during the process. They operate on a common centerline of rotation and are phased to rotate together. One headstock can function as a tailstock for allowing the preform to rotate as driven by the headstock. Attached to the headstock spindle is a chuck which clamps to the starter tube and the driven headstock spindle is hollow to allow passage of the tube or other apparatus through the bore. In current systems, the headstock/spindle is driven by a mechanical connection to a prime mover, such as an electric motor. The mechanical connection can be a shaft/chain/reducer, a timing belt/sprocket, or a reducing gear train. In all of these cases, the connection represents a maintenance/lubrication/wear concern. Also, the connections have an inherent backlash and stiffness associated with their operation which can result in uneven rotation of the starter tube, and which can affect the overall performance of the control loops and contribute to spindle prime mover to spindle commanded position error.
In addition to the foregoing problems, the even coating of the interior of the tube throughout its length is difficult to achieve. The gaseous coating material is introduced into the tube, under pressure, at, for instance, the tailstock end thereof and proceeds down the tube toward the headstock end. Especially in those instances where the tube is of considerable length, the velocity of the gaseous mixture decreases toward the headstock end, and there is a tendency for it to accumulate in that region, resulting in an unevenness of coating of the tube walls.
The present invention includes a direct drive spindle assembly which eliminates or alleviates the foregoing problems of the prior art. The assembly comprises a stationary headstock member supported on upstanding support legs mounted to the lathe and within which is mounted, with tapered roller bearings, a hollow spindle. A motor housing is mounted at one end of the headstock member and contains a circular array of motor windings. At the motor end of the spindle is mounted a reduced diameter portion which functions as the rotating motor armature, having winding and commutator magnets mounted thereon for interaction with the motor windings. At the other end of the spindle is mounted a chuck adapter and a chuck for grasping the primary exhaust tube portion of the starter tube. A bored magnetic encoder disk is mounted to the reduced diameter portion of the spindle and separated from the effects of the permanent armature and commutation magnets by shielding, and a circuit board having a detector mounted thereon is attached to the stator or motor housing for reading positional and rotation increment signals from the disk as it rotates with the spindle. The detector output can be applied to a processor which controls the motor power source to vary the speed of the motor, for example.
In accordance with one aspect of the invention, the spindle is rotationally sealed to the motor stator or housing, and the chuck adapter bore and the bore in the reduced diameter or armature portion of the spindle each have O-ring seals thereon so that when the primary exhaust tube and a secondary exhaust tube are inserted in the bores, the interior of the spindle constitutes a sealed chamber. In use, the primary exhaust tube attached to the end of the starter tube ends within the sealed chamber, and the secondary exhaust tube projects into the chamber and the primary exhaust tube through the bore in the armature portion of the spindle.
In accordance with another aspect of the invention, the motor housing has a circular passageway or groove formed on the interior wall thereof,, the walls of the passageway extending close to the armature portion of the spindle. Seals are mounted on either side of the passageway or groove to create a sealed space between the stator and armature portions. A bore extending from the exterior of the stator portion communicates with the groove for supplying a flushing gas, such as oxygen, to the groove, which gas is prevented from escaping by the seals. The armature portion of the spindle has a plurality of bores therein which communicate with the groove (or sealed space) and with the sealed chamber within the spindle, thus the flushing gas is directed into the sealed chamber of the spindle. In use, the end of the secondary exhaust tube which extends through the motor or armature bore to the outside of the apparatus is connected to a low pressure or vacuum supply through a rotatable connection so that the flushing gas is passed through a portion of the primary or outer exhaust tube and out of the secondary or inner exhaust tube, sweeping the remnant gases of the starter tube. In this way, the gaseous mixture in the starter tube is prevented from accumulating in the end region thereof, and more uniform coating of the starter tube walls is realized.
The spindle and direct drive motor, as will be apparent hereinafter, eliminates a large measure of jerkiness and uneven rotation, is of modular construction and lends itself to facilitating maintenance. The entire drive assembly can be quickly replaced as an entire unit, or individual sub-modules may be replaced without disturbing the remainder of the assembly. Other benefits realized with the invention, in addition to such simplified maintenance, are enhanced product quality, decreased associated equipment costs, flexibility of configuration, less production down time, and decreased process area contamination.
These and other features and advantages of the present invention will be readily apparent from the following detailed description, read in conjunction with the accompanying drawings.